Crude Pre-Heat Train exchangers are used to heat the crude oil as part of the distillation process. The crude is run on one side of tube-and-shell exchangers and heated by the hot streams run on the opposite side. More typically, crude oil is run through the tube side of the exchangers, however, some refineries run crude through the shell side with the hot stream on the tube side. The crude oil is run through a series of exchangers leading to the desalter and then to the atmospheric furnace. Whole crude oil fouling within exchangers is costly to the petroleum industry due to reduced throughput, energy losses due to needed increased furnace firing and higher cleaning and maintenance costs. In some cases, unplanned unit shut-downs occur due to fouling which adds to the high costs of fouling. To mitigate fouling addition of additives known as anti-foulant additives to crude oil before heat exchanger is a common practice.
Multi-purpose additives can reduce cost in the refining operation. Petroleum refineries incur additional energy costs, perhaps billions per year, due to fouling and the resulting attendant inefficiencies caused by the fouling. More particularly, thermal processing of crude oils, blends and fractions in heat transfer equipment, such as heat exchangers, is hampered by the deposition of insoluble asphaltenes and other contaminants (i.e., particulates, salts, etc.) that may be found in crude oils. Further, the asphaltenes and other organics are known to thermally degrade to coke when exposed to high heater tube surface temperatures.
Fouling in heat exchangers receiving petroleum-type process streams can result from a number of mechanisms including chemical reactions, corrosion, deposit of existing insoluble impurities in the stream, and deposit of materials rendered insoluble by the temperature difference (ΔT) between the process stream and the heat exchanger wall. For example, naturally-occurring asphaltenes can precipitate from the crude oil process stream, thermally degrade to form a coke and adhere to the hot surfaces. Further, the high ΔT found in heat transfer operations result in high surface or skin temperatures when the process stream is introduced to the heater tube surfaces, which contributes to the precipitation of insoluble particulates. Another common cause of fouling is attributable to the presence of salts, particulates and impurities (e.g., inorganic contaminants) found in the crude oil stream. For example, iron oxide/sulfide, calcium carbonate, silica, sodium chloride and calcium chloride have all been found to attach directly to the surface of a fouled heater rod and throughout the coke deposit. These solids promote and/or enable additional fouling of crude oils.
The buildup of insoluble deposits in heat transfer equipment creates an unwanted insulating effect and reduces the heat transfer efficiency. Fouling also reduces the cross-sectional area of process equipment, which decreases flow rates and desired pressure differentials to provide less than optimal operation. To overcome these disadvantages, heat transfer equipment is ordinarily taken offline and cleaned mechanically or chemically cleaned, resulting in lost production time.
There is a need to reduce precipitation/adherence of particulates and asphaltenes from the heated surface to prevent fouling, particularly before the asphaltenes are thermally degraded or coked. Such reduction will improve the performance of the heat transfer equipment, decrease or eliminate scheduled outages for fouling mitigation efforts, and reduce energy costs associated with the processing activity.
Antifoulant additives have been described in a number of commonly-owned applications, including U.S. Patent Application Publication Nos. 20110147275 and 20100170829, the disclosure of each of which is incorporated herein by reference in its entirety. However, there remains a need for alternative antifoulant additives capable of reducing precipitation and/or adherence of particulates and asphaltenes.
Antifoulant additives are associated with significant cost and other drawbacks, such that their indiscriminate use is advantageously avoided. Accordingly, a further need exists for methods of predicting a need for antifoulant additives according to the operating parameters of a refinery.